This invention relates to a wire electrode assemblage suitable for use in a variety of electrostatic apparatus and more particularly, to a wire electrode assemblage which will resist breakage under vibration and which is provided with arc-suppression means.
Electrostatic apparatus and methods are used in many industrial applications for applying a coating material to a substrate. One of the recent industrial applications of electrostatics is in placing a uniform lubricant coverage on a metal substrate at high speeds as disclosed in U.S. Pat. No. 4,066,803 to Scholes et al.
When such apparatus are placed on a continuous metal production line, or in other industrial production line settings, it is imperative that operation of the apparatus be reliable. Any breakdown, even for a few minutes, can cause stoppage of the line and require substantial time and expense in a subsequent start up.
One problem which has plagued some of these apparatus is breakage of the thin wire electrodes. This problem is especially significant where the wire electrodes vibrate, such as when an alternating current is imposed upon them.
A vibrating wire secured between two fixed points will undergo cyclic bending loads at each of the fixed points. If the load is of a sufficient magnitude to result in plastic deformation, a failure will occur after relatively few cycles. Assuming no plastic deformation, the wire electrode will still have a finite life depending on the fatigue life of the wire at a given magnitude of load cycle. Because the fatigue life in cycles increases as a logarithmic function of a decrease in the magnitude of the cyclic load, it will be appreciated that even a minor decrease in load will result in a substantial increase in fatigue life.
Another substantially contributing factor to fatigue at the point a wire is secured may be the method of securing the wire. If the method of securing results in crimping or plastically deforming the wire, a reduction in the cross-sectional area may occur which, given the same absolute magnitude of forces being applied to the wire, will result in a greater load per unit area at the fixed point and a logarithmically related shortened fatigue life.
Another problem which is sometimes encountered with wire electrodes is arcing from the sharp terminal end of the wire to ground. This is due to the much higher electric field generated at the terminal end of the wire than is generated along the length of the wire. Where such arcing occurs the electric potential which can readily be applied to the electrode may be limited to a level below that which is necessary for proper functioning of the electrostatic apparatus.